Heretofore, as a technique for sharing a key among a plurality of devices, a technique disclosed in Patent Document 1 has been known for example. In the conventional technique disclosed in Patent Document 1, in a system for user terminals having an access privilege to certain data to securely share a key for obtaining this access privilege, a management device holds encrypted key information for all user terminals, and when revoking the access privilege of a certain user terminal, it deletes the encrypted key information of this user terminal.
Moreover, Non-Patent Document 1 discloses one technique of a secret sharing scheme called a (k, n) threshold secret sharing scheme. A secret sharing scheme is a technique in which secret information is divided into several pieces that are distributed and stored, to thereby securely store the original secret information. In the (k, n) threshold secrete sharing scheme, provided that original information is divided into n pieces of information (shares) and stored, and k or more pieces of them can be obtained, the original data can be restored.
Moreover, Non-Patent Document 2 discloses a broadcast encryption type technique in which users eligible for subscribing contents on fee-based broadcasting are restricted to a limited number of users such as service subscribers. In the conventional technique disclosed in Non-Patent Document 2, a contents provider encrypts contents in a form to allow only a specified user group to decrypt the contents, based on key information distributed to all users.